Audience/user base for desktop application

Hi, I´ve built Humlix and it is slowly getting some downloads. The product makes it easy to test Web APIs in a new way.

One of my problems is that I really don´t know who is downloading and if they are using the product or not. I already have ideas on start tracking (anonymously) usage so I feel pretty comfortable with getting that in place soon.

Building a userbase
My big concern that leads to this post is that I want ideas on how to collect information about the user base to be able to email them about updates and news.
Of course, I have in the roadmap to have some kind of membership option that gives users some extra value, but until then I´m a bit blind.

I have a subscription box for the Humlix newsletter on the website and that is currently the only way for me to collect information about possible users so I can update them with the latest news. Very few choose to register for the newsletter.

The application has auto-update enabled so I can at least push out new features to existing users even though I don´t know who they are.

Do you have any ideas on nice ways of encouraging users to submit their email address or other contact information for a desktop application?